1. Field of the Invention
The present invention relates to high-precision positioning apparatuses, in holographic memories, for precise positioning of recording media relative to optical systems. In particular, the present invention relates to a high-precision positioning apparatus for precise positioning of a roughly positioned recording-medium by producing small movements of the recording medium.
2. Description of the Related Art
Memories widely used in, for example, computers, magnetically or optically write and read two-dimensional data to and from recording media. Examples of known magnetic memories include hard disks, while examples of known optical memories include CDs and DVDs. The quality of these memories has been significantly improved in terms of recording density so as to meet the demand for larger capacity. In addition, memories based on the hologram principle (hereinafter referred to as “holographic memories”) have been developed to further increase capacity.
In a holographic memory, an encoded object beam and reference beam create interference on a recording medium to form a hologram, thereby allowing information to be stored. The object beam is applied to the-recording medium in the form of two-dimensional digital data, while the reference beam is applied to the recording medium to play back the stored information in the form of two-dimensional digital data. The recording medium and an optical system including the object beam and the reference beam thus require precise positioning. This positioning is achieved by horizontal movements of the flat recording medium while the optical system is kept fixed. An example of such a holographic memory is disclosed in Japanese Unexamined Patent Application Publication No. 2001-93157.
Card-type or disc-type recording media are used in known holographic memories. In the case of a card-type recording medium, sliders are provided in an X-direction and Y-direction so as to move the medium, relative to the optical system, in the X-direction and Y-direction orthogonal to each other. In the case of a disc-type recording medium, a motor is connected to the center of the medium to rotate the medium, while a slider for traversing the motor is provided to radially move the medium.
However, a known positioning apparatus for a recording medium in a holographic memory has the following problem.
To randomly access information, large movements of a recording medium, on a page-by-page basis of a hologram, are required. On the other hand, for precise positioning of the recording medium relative to an optical system, small movements of the medium are required. For providing such small movements, stepping of a motor or a slider for driving the recording medium needs to be minimized. However, large movements-using such a driving unit place a heavy load thereon. Therefore, providing both large and small movements of the recording medium by the same driving unit is difficult and raises costs associated with the positioning apparatus for recording media.